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Best of John Robison

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Still more about the RNG

27 February 2006

Hi, John,

Thanks for your thoughtful answers to my previous questions. By using the phrase "pat answer" I didn't mean to imply that you were giving false information; but perhaps that the casinos, designers, and even the Gaming Commissions (well, perhaps only in Mississippi!) were misleading us all! Your sometimes response of "Still, only the casinos know for sure" worried me a bit; and I really did want to believe in the RNG and having a fair chance; and that's not what I'd been observing at the Grand and Copa Casinos in Gulfport, MS, where even the RNG's paltry payback as statistically "modulated" by the Telnaes map sometimes seems to be ignored.

Also the answer that Andrew Glazer gave in his book entitled Slots! How to Improve Your Odds at Winning (2001) did not ring true either. He claimed, e.g., that a machine is ordered by the casino (rated and tested) to payback a certain percentage (say, 90%) but that it may not achieve that rated payback for months or years; THUS some machines run hot (he mentioned a machine paying back at 102% for months) and some run cold. This had led me to the unfortunate use of the phrase "statistical aberration" and you rightly pointed out that "local non-randomness is an essential part of randomness." I had tested series-based RNGs for use in simulations on an IBM 7040 in 1961 at Auburn U. and would have thrown out immediately any RNG algorithm that was as bad as Glazer (a gambling columnist for the Detroit Free Press, at least in 2001) had indicated.

So back to square one. And you made me think about what I did know for certain: 1) I did NOT know whether the RNGs/ROMs were changed in the tournament machines at the Grand (they probably once were, when all 20 were put out of service for a week); 2) I DID KNOW that the players were told that the Double Diamonds were made "hot" and that all had an equal chance at winning; and, 3) I knew that most of the machines run quite cold most of the time since I play those machines weekly in normal service and had detailed payback rates (usually less than 90%, in individual cases as low as 50%) and frequency (averaging about 8 plays per payback) data on them. I had caught one "hot" machine (probably not reset after a tournament) and it paid back every 4 plays at a rate of about 230%.

So I finally found a friendly Slot Tech and I asked him how the tournament machines were made hot AND his answer was very interesting: in his words "the odd [physical reel] position numbers are ignored in tournament mode." Now there are 11 blanks and 11 non-blank positions on the reels of the Double Diamond. By ignoring blanks (or substituting a non-blank for a blank by simply bumping the index) the odds in your favor for a Double Diamond coming up on the payline would be doubled; and your chances of getting the triple Double jackpot would be increased by 8. The triple Double Diamond might come up once in 1,000 plays rather than the usual 1 in 8000 (I'm just making these numbers up since I don't have the Telnaes maps and I'm using 20 in the calculations, not 22).

A tournament round runs for seven minutes; a payback probably occurs every four plays (each play requiring generation of three numbers by the RNG) rather than the average of eight or so in normal play. Thus in one round each machine would give about 105 payoffs (I've counted many times) limited by the time to move the reels plus the time to get to the next payoff (four PLAY 3 CREDITS presses at about 1 second for most players). So with all machines in use the total number of plays would be 20 x 420 = 8,400 and you might expect up to 8 triple Doubles in the round. Sure, I know it might average out to that over the long run, and you might get only one or two which is usually observed in a round; and these numbers ignore how the virtual reels and Telnaes maps factor into this, and the odds are less than 1 in 8,000. Anyone carefully observing these tournaments would never play these machines!

One further comment: the typical total payback for a machine played optimally during a tournament usually ranges from 2500 to 4000, which tells more about the difference between RNGs than the players. The person who gets the jackpot (if any) of 2500 credits usually is the winner; unless beaten by the even luckier person who gets two triple doubles in the round . . . which occasionally happens and more frequently than you might expect, back-to-back.

Now making the machine hot in this way is what I would call "logic above the RNG" and opens up many other possibilities which I had been refusing to believe; but, as a programmer, could easily envision. You answered one of those: it's illegal to change the Telnaes maps under computer control (at least in most jurisdictions). Well, the maps weren't changed to make the tournament machines hot. Could it be that a simple "control parameter" like this could be set by the network; defaulting to the "cold" mode if the network or cluster controller were down? I assume the Gaming Commissions have many expert programmers on their staffs who would carefully analyze the control programs for such "logic above the RNG" tricks? Is that a good assumption?

One other nagging question lingers; a programming technique that would also qualify as "logic above the RNG." MacIntyre Symms in his book Slot Machine Strategy: Winning Methods for Hitting the Jackpot (2004)) defines "pay cycle" as a period when a machine is under its rated payback percentage and must pay back; and "take cycle" as the opposite. Administering this would require overriding the RNG at times. Is this actually done? And legal?

Thanks again for assisting me in developing my notes for a course in Slots Math/Engineering 101.

Jon

Dear Jon,

First, let me apologize for not responding sooner. I try to keep these "conversations" going by answering in a timely fashion, but your reply slipped through the cracks. Your original letter appeared about six months ago.

Now, let me address some of the statements in your letter.

You mention the "RNG's paltry payback." I just want to emphasize that RNGs do not have paybacks. All the RNG does is generate a series of numbers. It's the layout of the symbols on the virtual reel that determines the payback. I emphasize this point only because people sometimes think some RNGs are more generous than others, and the RNG really has nothing to do with determining the payback.

As for throwing out the RNG algorithm used in the machine Glazer described, you're confusing the performance of the machine (how quickly a machine approaches its long-term payback) with the performance of the RNG. You have to look at the numbers generated by the RNG to determine how well the RNG performs, not the payback of the machine. I'm skeptical that a machine paid back 102% for months, but Andy wouldn't have written it if it hadn't happened. We have to remember that just because an event is unlikely, that does not it is impossible.

I have had mixed experiences getting good answers from slot techs. I think a key point to remember is that techs are trained in the hardware of the slot machine and how to use the software running it. In most cases, they are not programmers. They may not necessarily know how the programmers have achieved certain results. I suspect what the programmers really did is create a set of virtual reels that do not have any blanks on them. This is a piece of cake, unlike altering the program to ignore the odd physical reel positions.

And I think this response indicates that the tech is not a programmer. How is the program to ignore those positions? What does it do if the number from the RNG indicates a blank should land on the payline? Get another result from the RNG or increment the physical stop to an even-numbered one? Ignoring the odd stops may be the effect, but he didn't say how that was achieved.

Any "logic above the RNG" is illegal in the United States. The more common term for this sort of thing is "secondary decision." The result indicated by the output of the RNG is what must be displayed — period.

Those definitions of "pay cycle" and "take cycle" are pure BS. Machines run hot and cold. That's true. But it is purely the result of random selection of outcomes. Overriding the RNG is illegal.

Best of luck in and out of the casinos,
John


Hi, John,

Just a few quick questions that came up as I read your recent articles:

  1. Are there any slots that use hardware RNG chips, or do they all use pseudo-RNGs that are started with a seed number and are deterministic?
  2. On a pseudo-RNG machine when will the sequence start over (when powered down, or reset by an attendant)?
  3. You mentioned the tournament machines — I had e-mailed you once before about the Double Diamond tournament machines at the Grand Casino in Gulfport, MS (no longer there and won't return! Although 3 casinos have reopened in Biloxi after Katrina) . . . I had speculated that it was perfectly legal (at least in Mississippi) to play the tournament machines, and that they were made "hot" by throwing a switch, and that an attendant had told me that in the "hot" mode the simple expedient of ignoring blanks caused them to be hot.

Any comments?

Thanks for your assistance. Hurricane Katrina has pretty well cured me of my gambling addiction!

Jon

Dear Jon,

I finally answered the e-mail you referred to in #3 above.

I don't know of any manufacturer that uses a hardware-based true RNG, but that doesn't mean that no one does. The manufacturers don't reveal many details about their RNGs. And I won't press them for an answer on this issue because it really doesn't matter.

The number of numbers an RNG can generate before it repeats is known as its period. Kilby and Fox, in Casino Operations Management, wrote that IGT's RNG generates over 4 billion numbers before repeating. That assumes that the RNG is not reseeded. A modern machine will reseed the RNG at certain times — exactly when is another detail manufacturers are reluctant to reveal.

John


Send your slot and video poker questions to John Robison, Slot Expert, at slotexpert@comcast.net. Because of the volume of mail I receive, I regret that I can't send a reply to every question. Also be advised that it may take several months for your question to appear in my column.

John Robison

John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots
John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots